Rapid eye movement (REM) sleep behavior disorder (RBD) is a REMsleep-related parasomnia, characterized by dream-enacted behaviors ranging from simple vocalizations or mumbling sleep talk to full-blown violent behaviors leading to injuries of the patient and/or the bed partner. The polysomnographic hallmark of RBD is the intermittent or sustained loss of the skeletal muscle atonia of REM sleep (REM sleep without atonia [RWA]), and this finding is mandatory in the diagnostic criteria (requiring polysomnographic [PSG] monitoring) in the International Classification of Sleep Disorders-2 (ICSD-2) (Box 1). RBD has attracted the attention of sleep researchers and sleep medicine clinicians for three reasons. First, this disorder provides insight about the mechanism of REM sleep generation in humans. Second, it affects the quality of life of the patients as well as their bed partnerswith social embarrassment, psychological conflict, and the possibility of injuries. Third, most RBD is associated with various neurologic disorders, especially with neurodegenerative diseases known as synucleinopathy, including Parkinson disease (PD), dementia with Lewy bodies (LBD), and multiple system atrophy (MSA). This last aspect of RBDhas also become amatter of interest for neurologists or movement disorder specialists, because RBD is often an antecedent of the onset (motor and/or cognitive manifestation) of synucleinopathy. This suggests future hope of neuroprotective treatment in some RBD patients who are considered in early stages of these neurodegenerative diseases.